This invention relates generally to the injection molding of articles with an aperture therein and, more particularly, to an improved gating apparatus for injection molding articles having large apertures.
Injection molding can be used advantageously to mold plastic articles of all shapes and description. Among such articles are those having an aperture therethrough, typically centrally located, such as in an audio compact disc or a lamp shade, or the like. Various apparatus are known in the art for accomplishing the molding of such articles, as shown in U.S. Pat. No. 4,368,028 to Grish et al., U.S. Pat. No. 4,530,654 to Rose and U.S. Pat. No. 5,423,672 to Gordon and Japanese Patent No. JP10-16005, each of which is incorporated herein by reference. These references disclose an injection molding apparatus having a nozzle with a central valve pin therein, creating an annular passage for melt to flow therearound to a gated tip. The valve pin permits the flow of melt to be positively selectively controlled and, when extended from the nozzle, also causes an annular ring xe2x80x98gatexe2x80x99 form between the pin and the nozzle tip, thereby permitting an annular article to be formed having a central aperture therethrough.
The apparatus of Grish et al., Rose, Gordon and/or JP10-16005, however, do not facilitate the forming of plastic articles having a large diameter aperture (such as a lamp shade), primarily for several reasons. Firstly, if the inner bore of the nozzle is enlarged to facilitate molding a larger diameter aperture, a greater volume of melt will remain in the nozzle after each cycle, thereby increasing the risk of melt degradation in the nozzle and increasing the difficulty in controlling the overall temperature of the melt. Also, guiding the valve pin can become a problem. U.S. Pat. No. 4,340,353 to Mayer, incorporated herein by reference, teaches a plurality of radially outwardly and angularly spaced extending arms 76, 78 used to guide a valve stem 74. These arms, however, together with flow opening 89, represent obstructions to the incoming flow of molten resin, toward the mold cavity, which generate several melt or flow lines in the finished product, decreasing the overall attractiveness of the product.
A possible solution to the problem of reducing melt volume in the nozzle is to increase the pin diameter correspondingly to reduce the overall volume of melt in the nozzle. If the pin diameter is so increased, however, the melt is exposed to an increased overall surface area in the nozzle which results in increased pressure losses in the runner system.
Another solution is posed by U.S. Pat. No. 5,785,915 to Osuna-Diaz, incorporated herein by reference, which discloses a nozzle having an outwardly flared bore and an outwardly flared insert that define between them a flared cylindrical melt passage. A valve sleeve surrounding the nozzle controls melt flow through an annular gate at the nozzle tip. This arrangement, too, would appear to suffer from the drawback of melt exposure to an enlarged surface area and resulting pressure losses in the runner system.
The prior art also proposes splitting or otherwise distributing the melt prior to delivery to the gate and injection into the mold cavity. For example, U.S. Pat. No. 5,324,190 to Frei, incorporated herein by reference, discloses the use of a plurality of borings 19 through the nozzle which break up the flow in the resin, as do the spacers 17. Similarly, U.S. Pat. No. 5,460,763 to Asai, also incorporated herein by reference, discloses a plurality of passages 21 for distributing the flow in the nozzle. In U.S. Pat. No. 4,394,117 to Taylor, incorporated herein by reference, though a central valve pin is not used, this reference does disclose a resin passage 55 which terminates in a conical dispersion head 66 mounted on its lower portion and a sleeve valve 30 fitted and slidably cooperating to selectively prevent the molten material from flowing into the mold cavity.
The devices of Frei, Asai and Taylor, however, can also result in the appearance of flow lines in the final product. To combat this problem, U.S. Pat. Nos. 5,783,234 and 5,840,231, both to Teng and incorporated herein by reference, disclose an even more complex apparatus to recombine the individual streams of molten resin after they are split and prior to entering the cavity gate, so as to minimize the appearance of flow lines. The apparatuses of Frei, Asai, Taylor and Teng, however, all require careful machining and make a resin colour change a laborious and time consuming proposition, as the intricate surfaces must be carefully cleaned before a new colour resin can be introduced.
Therefore it is desirable to provide an apparatus which permits improved control of the flow plastic melt from a hot runner system to a plurality of cavities to achieve more uniformity of formed articles. It is also desirable to avoid restrictions to or interferences with melt flow through the nozzle to permit the cavity to be filled with a uniform melt and receive a high quality product particularly for articles with large apertures therethrough.
In one aspect, the present invention provides an injection molding apparatus for forming articles having a hole, comprising:
at least one mold cavity formed between a cavity plate and an adjacent core;
at least one injection molding nozzle having an annular gate, the nozzle connectable to a source of molten material and capable of feeding molten material from the source to the gate through at least one melt channel through the nozzle, the gate communicating with the mold cavity and having a cross-section that is wider than the cross-section of the melt channel;
a valve pin disposed interior of the melt channel and the gate, the valve pin defining an unrestricted melt flow passage through the melt channel around and along the valve pin, the valve pin moveable between a closed position in which the valve pin substantially contacts the gate sufficiently to stop the flow of molten material through the gate, and an open position in which molten material can flow unrestricted to the gate.
In a second aspect, the present invention provides an injection molding apparatus for forming articles having a hole, comprising:
at least one mold cavity formed between a cavity plate and an adjacent core;
at least one injection molding nozzle having an annular gate, the nozzle connectable to a source of molten material and capable of feeding molten material from the source to the gate through at least one melt channel through the nozzle, the gate communicating with the mold cavity and having a cross-section that is wider than the cross-section of the melt channel;
a valve pin disposed interior of the melt channel and the gate, the valve pin defining an unobstructed melt flow passage through the melt channel around and along the valve pin, the valve pin moveable between a closed position in which the valve pin substantially contacts the gate sufficiently to stop the flow of molten material through the gate, and an open position in which molten material can flow unobstructed to the gate.
In a third aspect, the present invention provides an injection molding apparatus for forming articles having a hole, comprising:
a mold having a cavity plate and an adjacent core which enclose a mold cavity therebetween;
an injection molding nozzle having a melt channel therethrough, the melt channel communicating with the mold cavity through an annular gate at the tip of the nozzle;
a valve pin disposed interior of the melt channel, the valve pin and the melt channel defining a melt flow passage around and along the valve pin, the valve pin having a head portion adjacent the nozzle tip and a stem portion remote from the nozzle tip, the head portion having a wider cross-section than the stem portion; and
an actuator operatively linked to the stem portion of the valve pin to move the valve pin between an open position with its head portion adjacent the gate in which molten material can flow through the gate into the mold cavity, and a closed position with its head portion blocking the gate to seal the communication between the nozzle and the mold cavity.
In a fourth aspect, the present invention provides an injection molding system for forming articles having a hole, comprising:
a mold cavity plate and a plurality of mold cores defining with the mold cavity plate a plurality of mold cavities;
a melt distribution manifold for delivering molten material to the mold cavities;
a plurality of injection molding nozzles respectively associated with the mold cavities, each nozzle having a melt channel therethrough, the melt channel communicating with its respective mold cavity through an annular gate at the tip of the nozzle;
each of the nozzles having a valve pin disposed interior of the melt channel, the valve pin and the melt channel defining a melt flow passage around and along the valve pin, the valve pin having a head portion adjacent the nozzle tip and a stem portion remote from the nozzle tip, the head portion having a wider cross-section than the stem portion; and
actuating means operatively linked to the stem portion of each of the valve pins to move each valve pin between an open position with its head portion adjacent the gate in which molten material can flow through the gate into the mold cavity, and a closed position with its head portion blocking the gate to seal the communication between the nozzle and the mold cavity.
The valve pin preferably has a smooth transition portion between the stem portion and the head portion. The distal end of the head portion may have a guide portion which engages the core for guiding the valve pin between its open and closed positions. A core sleeve may be provided for engaging the perimeter of the head portion of the valve pin. Further, the perimeter of the head portion of the valve pin may form part of the surface of the core when the valve pin is in its closed position to at least partly define and form the hole.
The shape of the annular gate (and the parts that form the gate) may be chosen to form a hole of any desired shape in the articles to be molded. Thus, e.g., the gate cross-section may be circular, oval, square, rectangular, or irregular.